Mac not pairing to the wireless keyboard following upgrade

We have an older Mac (2009) with a wireless keyboard. Unfortunately after the recent update to Yosemite 10.10.3, there was a small issue concerning pairing upon restart. The green light would flash for a few minutes, but the Mac simply refused to recognize it.

Despite several mundane efforts (changing batteries, unplugging/replugging all connections, etc.) I could not log on. I also ensured that all other gadgets in the immediate vicinity had Bluetooth turned off. The only advice provided on these pages as far as I could glean from my phone was to simply go in under System Preferences and tweak the Bluetooth settings. I would have gladly done so, had I been able to even log in.

The end result is that I went to a local non-Apple affiliated electronics store and purchased a garish USB-connected gamer keyboard for an over the top sum of money. Design-wise, it is not compatible with the sleek minimalist Apple design, yet it was the only USB keyboard available which supported OS X. A plug-and-play wonder, it got me through the Forgotten Password? (no, but that's where I ended up) screen, and once logged on, the old Apple wireless keyboard pulled a Lazarus on me and is now up and running without me having to mess around with the Bluetooth settings.

Have others experienced this problem following the update?

If so, what other possibilities are there to log on?

There must be a simpler (and less costly) solution than simply buying a hideously ugly new keyboard which my tween son will drool over. The solution I came up with worked, although it is irritating to discover that most keyboards on sale in the market cater solely to the Windows universe, and thus incompatible with OS X unless you move up in to the Android Dungeon-quality nerdfest keyboard.

Thanks!